The global positioning system (GPS) is the most known system for positioning of a movable unit, such as a car, ship, or generally any entity or person provided with a positioning signal receiver. As is well known, GPS involves a plurality of satellites which orbit the Earth and transmit GPS ranging signals which are captured and analyzed by the GPS receiver to determine a current position with respect to the surface of the Earth.
Assisted GPS (A-GPS) positioning is an enhancement of GPS, specifically designed to facilitate integration of GPS receivers in mobile terminals for cellular communication systems. A number of GPS reference receivers are attached to the cellular communication system and serve to collect assistance data that, when transmitted to GPS receivers in mobile terminals connected to the cellular communication system, will enhance the performance of the terminals' GPS receivers. With A-GPS the search for GPS satellites may be rendered much faster in the GPS receiver, which is advantageous in devices like mobile terminals where the GPS functionality may not be constantly activated. When the GPS receiver starts up it will use the assistance data to determine an approximate indication of its current position and thus concentrate its search to certain satellites only rather than all satellites in the GPS system. A-GPS accuracy can become as good as 10 meters also without differential operation. The development of A-GPS was accelerated by the U.S. FCC's North American E-911 positioning mandate, requiring that the location of a cell phone be made available to emergency call dispatchers.
However, the integration of GPS and cellular telecommunication to obtain A-GPS has caused some problems. For instance, certain aspects of the 3GPP W-CDMA standard (which is the basis for the UMTS systems of today) have been found to impair end to end performance, in particular in North American E-911 positioning. Problems are caused by a too vague specification and signaling of what is meant by positioning accuracy. To understand this it is first noted that accuracy in the radio navigation field, in particular for A-GPS, is a random quantity. This means that any time a position uncertainty is determined, the uncertainty must be accompanied by a corresponding probability that the terminal is actually in the region defined by the reported position and the reported uncertainty. In the 3GPP W-CDMA specification uncertainty can be expressed in terms of an uncertainty circle, an uncertainty ellipse and an uncertainty ellipsoid. More specifically, one problem affecting A-GPS originates from
1. The fact that the 3GPP standard does not specify exactly how the uncertainty and the probability (also known as the confidence) are related. Typically this would require a specification stating how the confidence value shall be related to the covariance matrix of the uncertainty measure. In particular, this would require a different handling of 2- and 3-dimensional uncertainties.
2. The fact that the requested horizontal and vertical accuracies, that are received in the radio access network (RAN) from the central network (CN) (and the end user), do not specify the confidence for which the quality-of-service (QoS) request is valid.
3. The fact that the horizontal and vertical accuracies, that are transmitted from the radio network controller (RNC) to the terminal to specify the requested accuracy of the measured A-GPS result, are not accompanied by a confidence value.
4. The fact that the A-GPS result reported from the terminal to the RNC of the W-CDMA RAN normally contains horizontal accuracy and a corresponding confidence value, most often also a vertical accuracy.
5. The fact that shape conversions are applied in the RNC of the W-CDMA RAN, e.g. in order to scale the accuracies according to operator configured confidence values for reporting of positioning results.
Therefore, when the RNC of the W-CDMA RAN shall provide the terminal with requested accuracy information, the RNC neither knows nor can signal a correct confidence value to the terminal. As a consequence, the action in telecommunication systems known from the prior art has been to forward the incoming horizontal and vertical accuracy requirements to the terminal unaffected. Then the terminal tries to provide a result according to the request; however, this result is provided at the confidence level selected by the specific terminal type, not necessarily at the confidence level really needed by the end user. Therefore, the RNC of the W-CDMA RAN may perform a shape conversion to scale the obtained horizontal and vertical accuracies to the level that is configured for the specific service (e.g. as directed by the Client Type IE). It may then happen that the terminal provides a result, exactly at the received requested accuracy level, with the accuracy requested, and at a confidence level of typically 39% or 20% (2D and 3D covariance matrix level, respectively). In case of North American emergency positioning, 95% confidence is however required for reporting. Hence, in this case, the RNC would scale up the obtained horizontal and vertical accuracies, a fact that would result in a failure to meet the requested accuracies that were originally received from the CN and the end user. The fact that the QoS was not fulfilled may also be signaled to the end user, and it may also affect performance management counters. Both of these effects will yield statistics that in the end may be reported to the federal communication commission (the FCC) with the result that the operator's fulfillment of the regulatory E-911 requirements may be questioned.
The present inventors have realized that there is room for improvements with respect to these problems.